Waterosity 2009:Green Roofs at the University of Minnesota's Landscape ArboretumJonathon HensleyUniversity of Minnesota Graduate Student

The dense, accelerated pace of modern urban development has affected many of the earth's natural processes. Asphalt and concrete rooftops, roads, and parking lots cover up to seventy percent of land area in dense cities like New York, while open space in sprawling cities like Phoenix, Arizona is lost to development at a rate of 1.2 acres per hour.

Approximately 1.5% of the continental United States, an area roughly equivalent to the state of Ohio, was covered by impervious surfaces in 2004. This percentage continues to grow and can be as high as 75% in urban areas. Of those impervious surfaces (not allowing the permeation of water), roofs can constitute a significant percentage. Such growth in impervious surfaces can result in a variety of environmental impacts including reduced aquifer recharging, overwhelmed storm water systems, urban heating, decreased surface water quality, and increased air ozone and particulate concentrations. These negative environmental effects impact residents and municipalities by affecting clean water availability, storm water and sewage infrastructure costs, decreased runoff water quality, decreased employee productivity, decreased wildlife habitat, and increased operating costs of buildings through increased heating and/or cooling costs. Alternative solutions to traditional impervious building methods are being sought in order to mediate these negative environmental impacts and reestablish the green spaces desperately needed in our metropolitan spaces.

State of Maryland: Dept. of the Environment

Green roofs are becoming an increasingly popular alternative to traditional roofs in metropolitan/suburban areas to reduce the environmental impact of impervious building roofs and to increase green spaces in downtown metropolitan areas. In fact, they are being used to increase green spaces across all areas of commercial development. The above illustration identifies the significant impacts of land development on a local hydrologic cycle; site development substantially increases surface runoff and the removal of tree canopy interception strips away the natural buffering capacities of green spaces- this increase in surface runoff and the removal of green space buffering has had far reaching consequences for our densely populated metropolitan spaces.

The environmental impacts and/or benefits of green roofs in this scenario can include decreased water and nutrient run-off from developed land, reduced ambient air temperature, reduced atmospheric CO2 levels, new garden areas for people and wildlife, and reduced airborne particulates and noise pollution in congested urban environments. Financial benefits can include reduced energy costs for buildings, reduced city infrastructure needs/costs, and improved psychological well-being of citizens resulting in increased productivity among employees. As roofs can account for up to 15-30% of impervious surfaces in cities, the impact of green roofs on the urban environment and citizen well-being can be high.

Extensive Green Roof Ufa Fabrik, Berlin, Germany

Green roofs can be built in a variety of contemporary styles; the most prominent distinction is between extensive and intensive systems. Intensive green roofs have been used in our country longer than extensive green roofs, but their reliance on deep soil media profiles and irrigation/fertilization to support traditional landscape plants has limited the progress and general utility of this style. By comparison, extensive systems rely on profiles generally not more than six inches in depth and have shown great merit for retrofitting existing buildings that do not have the appropriate structural support for an intensive system.

Extensive green roofs are composed of several material layers designed to protect a building and offer functionality in a metropolitan space. These layers are often composed of waterproofing membranes, a root impermeable layer, a drainage layer, extensive growing media, and vegetation. These systems have been shown in independent studies to positively impact urban/metropolitan environments by re-establishing green biological buffers that mediate storm water runoff, capture particulate pollution, reduce building energy costs for heating and cooling, provide sanctuary for urban wildlife, and improve citizen morale.

For more information on this topic, please see the following references and links: